Bass Missing Down the Middle

While reading through a professional audio forum online, a question was brought up by someone regarding the bass response of his system. He was having problems with very little bass down the middle of the room, but still had bass on the sides. On one side of the stage, he had to put the subwoofer behind the main speakers, but he had delayed the rest of the system to compensate. He was unsure if the small delay was causing the low bass in the middle issue. His system was comprised of very high quality components with the proper amplifiers and loudspeaker processors, although no Carvin Audio products.

He is definitely not the first person to ever have this happen.  So I thought, time for a blog to let the Carvin Audio fans know what was going on, in case you ever experience this issue.

To start with, let's look at a stereo set of subwoofers placed 10 meters apart with a 2ms delay on one side. I do not know the exact delay that the person posting the question used, but based upon the speakers he was using 2ms works as an good approximation here.

The top subwoofer in this image is delayed two ms and you will notice that there is a distinct power alley right down the middle with a couple of nulls just to the sides of the power alley. A closer examination shows that the coverage is slightly steered towards the side with the delay. This shows that the lack of bass down the middle is NOT caused by the delay, but it does show a system technician can slightly steer bass using delay.

Now we have confirmed the delay is not the issue. So what could it be? Having dealt with odd anomalies over the years, I had experienced this with several installs at various venues. I knew exactly what the problem was.   Where to find the problem cause in order to fix it is another issue unto itself.

In the next image all of the settings are the same, except I reversed the polarity of one sub. This is common and I have fixed this issue in many venues. We now have two subwoofers with one delayed 2ms and one with reversed polarity.

Looking closely at the image, you'll immediately recognize that the bass is still steered towards the sub with the delay, but this time the power alley is gone. Instead, there is a distinct null right down the middle of the sound field. We have now confirmed the cause probably is a subwoofer that has the polarity reversed.

The problem may exists for several reasons. The most common cause of this is the operator probably made or fixed his own speaker cables and accidentally attached the pins on the Speakon connector incorrectly on one side, causing the positive lead on one to be connected to the negative lead on the other side and vise versa.  Another possibility is that the subwoofers were being run by bridged amplifiers using the hard wired terminal or banana plugs that were reversed. Another common cause is someone accidentally hit the polarity reverse setting in the loudspeaker processor. A final possibility, although very unlikely, is either the manufacturer or someone maintaining the speakers accidentally connected the leads inside the cabinet to the driver backwards, again causing a reversed polarity.

So, if you are ever finding a lack of bass right down the middle, take a quick look at your processors and also double check your cables for proper polarity. Furthermore, by purchasing your cables pre-made, or fully testing repaired cables pin for pin will prevent a lot of issues with these types of anomalies.

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